The present invention relates to a rotary hearth furnace (RHF) in which premixed pellets of metal oxides and carbon are transported on a radially moving belt during which time the pellets are exposed to hot combustion gases in a countercurrent manner resulting in the metal oxide being reduced to sponge metal. More particularly, the present invention relates to such a rotary hearth furnace in which oxygen is injected into the furnace such that the carbon monoxide and hydrogen present in the combustion gases are oxidized to generate additional heat for heating the pellets. The injection of the oxygen is accomplished by using an oxy-fuel burner having a flat, wide, flame to increase heat transfer to the pellets. The oxygen is injected at strategic locations within the furnace to combust the exhaust gases and still allow the reduction of the metal oxide.
Doughnut shaped rotary hearth furnaces used to reduce metal oxides are well known in the art. In a traditional rotary hearth furnace heat is provided by air fuel burners placed in the furnace hearth aimed at the top of the pellet bed. The radial belt inside the furnace moves around the circumference of the furnace at a rate sufficient to enable the pellets to reside in the furnace for approximately one complete revolution. Combustion gases are exhausted through a top vent near where the pellets are introduced. Combustion gases in a rotary hearth furnace are often generated from other sources such as the offgas of a smelter.
It is important that the combustion takes place in the most thermally efficient manner in order to conserve both fuel and oxidant as well as in a manner to reduce harmful emissions. It is also important that the furnace atmosphere is sufficient to reduce the metal oxide. However, in conventional rotary hearth furnaces such as described above, there is a loss of efficiency and unwanted emissions due to the partial products of combustion being exhausted out of the furnace. As will be discussed, the present invention provides improvement to rotary hearth furnaces such as those discussed above by utilizing additional oxygen to oxidize certain post-combustion gases which provides additional heat; and to reduce carbon monoxide in the exhaust, while maintaining an appropriate reducing environment at all locations in the furnace so that accelerated oxide reduction can occur.